(a) Field of the Invention
The present invention relates to thermosetting resin compositions which are excellent in heat resistance and non-flammability and useful as materials of high functional molding materials, paint, coating materials, adhesives, sealing materials, prepreg for printed wiring boards, metal-clad laminates, FRP and carbon goods. The present invention further relates to cured products of the thermosetting resin compositions and prepreg, metal-clad laminates and wiring boards which are produced by using the thermosetting resin compositions.
(b) Description of the Related Art
Thermosetting resins, such as phenolic resins, melamine resins, epoxy resins, unsaturated polyester resins and bismaleimide resins, have been used in various industrial fields because of reliability and heat resistance due to the thermosetting properties thereof. These resins however have individual disadvantages that, for example, phenolic resins and melamine resins generate volatile by-products on curing, epoxy resins and unsaturated polyester resins have poor heat resistance and bismaleimide resins are very expensive, and these disadvantages should be put up with according to the usage. So there have been developed new thermosetting resins free from such disadvantages.
An example is the development of dihydrobenzoxazine compounds (Japan Patent Application Unexamined Publication No. 49-47387 and the specification of U.S. Pat. No. 5,152,939). Since the compounds are cured with heat by the ring opening polymerization of dihydrobenzoxazine rings, volatile matters are hardly generated during curing.
Researches in the curing and reactivity of the compounds have been reported by Burke et al. in J. Am. Chem. Soc. 3424(1956) and by Riess et al. in Polym. Sci. Technol. 27(1985). Japan Patent Application Unexamined Publication No. 7-188364 discloses dihydrobenzoxazine compounds which are improved in curability by leaving a specific percentage of phenolic hydroxyl groups uncyclized.
The dihydrobenzoxazine compounds disclosed in Japan Patent Application Unexamined Publication No. 49-47387 are cured by ring opening polymerization to give cured resins which are superior to the cured products of conventional thermosetting resins in heat resistance and strength. The curing by ring opening polymerization, however, is time consuming as compared with the conventional curing of phenolic resins, and has limited usage due to the low productivity.
As disclosed in Japan Patent Application Unexamined Publication No. 7-188364, dihydrobenzoxazine compounds containing in molecules a specific percentage of phenolic hydroxyl groups left without converting to dihydrobenzoxazine rings are improved in curability, but are difficult to synthesize steadily since ring opening polymerization proceeds by the heating during the synthesis of the compounds.
Although Burke et al. and Riess et al. reported improvement of curability by the addition of monofunctional phenolic compounds, the improvement is insufficient, and a decrease in heat resistance and mechanical strength is caused by a lowered crosslinking density.